<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      margin-bottom: 20px;
    }
    .circle {
      background-color: red;
      width: 100px;
      height: 100px;
      border-radius: 50%;
    }
    .triangle {
      width:0;
      height:0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-bottom: 100px solid red;
    }
    .trapezoid {
      width: 100px;
      height: 0;
      /* border-left: 100px solid transparent; */
      border-right: 100px solid transparent;
      border-bottom: 100px solid red;
    }
   
  </style>
</head>
<body>
  <div class="box">
    <div>圆形</div>
    <div class="circle"></div>
  </div>
  <div class="box">
    <div>三角形</div>
    <div class="triangle"></div>
  </div>
  <div class="box">
    <div>梯形</div>
    <div class="trapezoid"></div>
  </div>
</body>
</html>